1. Field of the Invention.
This invention relates to a composition and method for synthesizing high temperature superconductors in bulk and thin film.
2. Discussion of the Related Art.
Superconductivity above 130 K has recently been observed in the multiphased Hg-Ba-Ca-Cu-O (HBCCO) compound system. This multiphased HBCCO was found to consist of the homologous series HgBa.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+2+.delta. [Hg-12(n-1)n]with n=1, 2, 3, . . . and superstructures of well-defined stacking-sequences of members of Hg-12(n-1)n. Similar to the TlBa.sub.2 CA.sub.n-1 Cu.sub.n O.sub.2n+3+.delta. [Tl-12(n-1)n]structure, Hg-12(n-1)n has a general layered structure with repeated stacking of the rock-salt type layers of (BaO)(HgO.delta.)(BaO) and the perovskite-type layers of (CuO.sub.2)Ca(CuO.sub.2) . . . Unlike T1-12(n-1)n, which contains the trivalent T1 and therefore the rock-salt type layers of (BaO)(TlO)(BaO), Hg-12(n-1)n has the divalent Hg and thus the oxygen-deficient rock-salt type layers of (BaO)(HgO.sub..delta.)(BaO).
Soon after the reports of superconductivity above 130 K in the multiphase Hg-BaCa-Cu-O (HBCCO) samples, efforts were made to identify the compound phase or phases responsible for superconductivity at such high temperatures. Very recently, HgBa.sub.2 CaCu.sub.2 O.sub.6+8 (Hg-1212) and HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.8+8 (Hg-1223) were successfully synthesized and characterized. Hg-1212, which has two CuO2-layers per unit cell, displays a tetragonal structure with lattice parameters a=3.862(1) .ANG. and c=12.707(5).ANG., and a magnetically-determined sharp superconducting transition at .about.112 K before oxygenation and up to .about.120 K after oxygenation.
On the other hand, Hg-1223, which possesses three CuO.sub.2 -layers per unit cell, exhibits an orthorhombic structure with lattice parameters a=5.451(2) .ANG., b=5.432(2) .ANG., and c=15.826(7) .ANG., and a magnetically-determined sharp superconducting transition at .about.120 before oxygenation and .about.135 after oxygenation. The resistivity of Hg-1223 becomes zero at 134 K which is the highest reproducible resistively determined transition-temperature (T.sub.c) ever observed. The irreversibility line H.sub.i (T.sub.i) of Hg-1212 was carefully examined in the art and was found to follow the power law H.sub.i .varies.(1-T.sub.i /T.sub.co).sup..alpha. with .alpha..about.5/2 and T.sub.co being the onset superconducting transition-temperature in the absence of a magnetic field.
A similar power law was also observed in the art to apply to YBa.sub.2 Cu.sub.3 O.sub.7 (Y-123) and the double BiO.sub.2- and TIO.sub.2- layer high-temperature superconductors (HTS's) Bi.sub.2 Sr.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 and Tl.sub.2 Ba.sub.2 Ca.sub.n-1 Cu.sub.n O.sub.2n+4 where v=2 or 3, but with n.about.3/2 and .about.11/2, respectively. This suggests that the critical current density (J.sub.c) for Hg-1212 should lie between those of Y-123 and the Bi/TI-based HTS's at the same reduced temperature, due to the stronger coupling between CuO.sub.2 -blocks.
Given the higher T.sub.c to comparison with that of Y-123, Hg-1212 may be a good candidate for large total current (I) or large J.sub.c applications provided that the compound can be made into practical forms and the compound stability is improved. One such practical form is important for electronic applications is thin film.
The standard solid-state reaction technique has been employed to prepare the superconducting phases of the HBCCO system. Oxides and/or carbonates of the cations have been used successfully as the starting ingredients. Because of the low decomposition temperature (.about.500.degree. C.) of HgO with respect to the high reaction-temperature (.about.800.degree. C.), and the high vapor-pressure and corrosive nature of Hg at the reaction temperature pose a serious challenge to the preparation of Hg-1212, Hg-1223 or other homologous members. The absence of Ca in HgBa.sub.2 CuO.sub.4+8 (Hg-1201) makes the preparation of Hg-1201 slightly easier. Other problems include the easy formation of the insulating layered compound of CaHgO.sub.2 at .about.500.degree. C., which is low compared with the reaction temperature for the HBCCO compounds, and the easy reaction of the precursor with moisture and CO.sub.2 in the air.
The unusual linear or dumb-bell coordination of Hg in HBCCO as demonstrated in HgBa.sub.2 RCu.sub.2 O.sub.6.5+.delta. where R=rare earth and Hg-1201 show that there should exist a large number of vacant oxygen-sites in the Hg-layer. An even higher T.sub.c may be found in the HBCCO system by finetuning the doping levels, especially the anion doping. To carry out effectively the study, one needs samples of pure or nearly-pure phases of Hg-12(n-1)n. Unfortunately, the complexity of HBCCO compound chemistry and the toxicity of the Hg-containing ingredient make such a task extremely difficult.